The Blockchain
                     The blockchain will disrupt
the way we make 
transactions
A blockchain is a decentralized 
chained list of immutable 
blocks of data
Decentralized
chain of blocks
of data
A blockchain for what ?
For storing:
1. documents (shares, mortgages, votes, ...)
2. money transactions
3. seller-buyer exchanges (tracability)
For running “smart contracts”: 
a computerized transaction protocol 
that executes the terms of a contract.
A blockchain for whom ?
What is a block ?
Index: 
the order of the 
block in the chain
Timestamp: Data: 
the creation date of a file, a text, a money 
the block transaction, ...
What is a block ?
(continued)
Hash: 
the signature of a The nonce: 
content a number used during the 
hash validation (mining)
What is a hash ?
What is a nonce ?
Number used ONCE
Validating a hash
On some blockchain (like Ethereum), the 
difficulty is related to “puzzle” mining, another 
concept of difficulty.
What is mining ?
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Mining (1) 
One miner finds nonce
Transaction requests 
mining
Mining (2)
Send nonce to 
users to 
check validity
Verified !
Reward
Proof Of Work - Proof Of Stake
Proof of Work Proof of Stake
A piece of data that requires A concept that allows ‘mining 
amount of computation, but power’ according to the 
can be easily checked. miner’s bitcoin cash level.
Classical way of mining. Quicker & based on trusted 
miners.
All miners on the same level.
Miners are not on the same 
level.
Building a chain of block: an example
1. The genesis block
2. Adding another block
The genesis block
Not signed ? = Not valid !
Adding a new block
New block not signed ? 
= The chain is invalid !
Adding a new block: after mining
Blockchain immutability
A block is immutable
● no edition
● no deletion
Blockchain PROs
4. Robustness
1. Disintermediation
2. Quicker run 
of contracts 
3. Lower transaction 
costs
Blockchain CONs
1. Immutable = no deletion = 
engraved for life
2. Immutable = no edition -> 30 TWh (2017)
dangers for smart contracts = Ireland 1 year electricity
= production of 4 nuclear plants
3. Mining = CPU consumption & 
dedicated hardware compared with
4. Smart contracts need good Banks (2017) = 100 TWh
specs and good testers Web (2013) >= 200 TWh
Gold mining (2017) = 240 TWh
The “51%” attack
About the smart contract
Smart Contracts are IT protocols that facilitate, verify and execute the 
negotiation or execution of a contract
Private vs public blockchains
Private blockchain Public blockchain
1. Faster 1. Plenty of mining nodes 
= slower
2. Scalable: few nodes, more
transactions 2. Plenty of mining nodes 
= more secure
3. Few nodes = less secure
3. Not need to trust anyone
4. Centralization

Credits
1. Live demo of blockchain: https://blockchaindemo.io/
2. Introduction à la technique de la blockchain: https://blog.octo.com/introduction-technique-a-la-blockchain/